Galvanizing of continuous steel sheet is normally carried out by dipping said steel sheet in a bath of liquid metal, usually zinc, aluminum or mixtures thereof along with other additives, in order to form a thin coating of metal on said sheet.
Galvanizing plants usually process continuous steel sheet which is uncoiled at one end of the plant and is cleaned, prepared and dipped in the hot liquid coating metal, cooled down and recoiled at the other end of the plant. All of the equipment and process is designed and operated so that the quality of the surface of said sheet is optimal and with the aim of processing great lengths of steel sheet without interruption.
Most of the galvanizing plants, currently under operation, use two channel type electric induction pots to hold a bath of hot liquid coating metal (zinc or aluminum based coatings), one of the pots in operation and the other in stand-by position. These types of induction pots cannot be frequently emptied and refilled, because as a result of their design the induction coils would be damaged and lost. Consequently, they are always kept hot filled with liquid metal, consuming electricity during the stand-by periods in the order of 8,400 kwh per day for a pot with a capacity of about 100 metric tons. The continuous heating of both pots can last for a number of years. The common practice is to carry out monthly production runs to produce coated sheet with two different alloys according to the production schedule. Duration of each run per alloy varies normally between about 5 and 15 days. Some plants also have a pre-melt furnace, of the gas fired type or of the electric induction type for melting solid ingots of zinc to fill the operating pots. Another variation of the prior art (with off-set melting by coreless induction furnaces) is shown in U.S. Pat. No. 5,354,970 (which is incorporated herein by reference). The flexibility of the current galvanizing plants for carrying out short runs or frequently changing the metal coating is very limited and the two-pot arrangement is expensive (both in duplicative capital cost and in added standby heating costs). In order to change the coating composition, the operating pot is moved to the stand-by position and the other pot is moved to the operating position. Another alternative is to move the strip being immersed into the operating pot to a second position where it is immersed in the other pot. If three or more different coatings are desired, the practice of the prior art to have one separate pot for each different coating composition is not practical and would be very expensive.
The present invention overcomes the disadvantages of the prior art and uses a single dip pot of the coreless type, which is also capable of melting solid metal ingots and of forming the required galvanizing liquid bath rapidly without the necessity of maintaining a second galvanizing pot in stand-by (with the consequent expense of electric energy and the other financial costs of a second pot). If more than three different coating compositions are desired, this invention provides a way of having solid ingots of each desired coating ready to be used, while in the prior art practice only two different compositions at a time were contemplated.